Add {margin} directive

[choldgraf]

Context

THe documentation seems to suggest that the {margin} directive is supported, but the following usage of that directive does not lead to anything in the margin of the page:

Here's some:

```{margin}
Test marging content
```

I noticed this when trying to add documentation about it to mystmd.org/guide.

Proposal

Figure out why {margin} content doesn't work in the book theme.

For example, it is mentioned in this issue too:

• Hide TOC when it intersects with margin content #170

Tasks and updates

No response